Ceramic materials based on silicon nitride and silicon aluminum oxynitrides have been the subject of much research over the last decade due to their engineering potential. Materials have been developed with high strength, creep resistance, hardness, resistance to chemical attack, and with retention of these properties at high temperatures. Due to their high hardness, machining of these ceramic materials once densified involves costly, difficult diamond machining. Hence much research has also been conducted into near-net shape forming and densification. For complex shapes, this does not permit hot pressing. Instead, sintering and hot isostatic pressing technologies have been developed. Also, commercially, sintering and hot isostatic pressing methods of densification are to be preferred over hot pressing for mass production since for hot pressing the productivity is poor and the production cost high.
This invention relates to one of the silicon aluminum oxynitrides (sialons) which has the alpha silicon nitride crystal structure but of expanded cell dimensions commonly known as alpha sialon. This material has been known for a number of years as a potentially useful engineering ceramic. Literature reference report full densification of these materials by hot pressing only, along with attempts at full densification by sintering without the application of pressure which yielded products of low strength. One such reference is an article entitled "The Strength of Alpha Sialon Ceramics", Journal of Materials Science, 15 (1980).
There are no references to the use of densification additives for these materials.
There is little or no evidence of high densities, that is, near theoretical densities or of high strengths being sustained in production artifacts having greater than about 70 weight percent of crystalline phases as alpha sialon. Little attention has been given to the usefulness of such products as enginering ceramics compared to the beta sialons or to manufacturing methods for producing such products which allow commercial mass production or the densification of complex shapes, that is, which avoid hot pressing. However, prior to this invention beta sialons and mixtures of alpha and beta sialons having less than about 70 weight percent as alpha sialon have found application as metal cutting tools as described in U.S. Pat. Nos. 4,323,323 and 4,563,433 and have been commercially manufactured for this use.